Organic photovoltaic (OPV) cells have seen remarkable progress in the past few decades. OPV devices using a bulk heterojunction (BHJ) based photoactive layer, positioned between an electron-donating semiconducting polymer and an electron-accepting fullerene derivative, are one of the most promising systems in this field. The most well-investigated BHJ based polymer solar cells consists of poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) blend with a power conversion efficiency (PCE) of 5%. In polymer solar cell technology, chemical tailoring allows an endless possibility of engineering a semiconducting polymer with appropriate absorption and charge transport properties. In the last few years, there are varieties of novel low band gap semiconducting polymers, such as 2,7-carbazole based copolymers, and polythiophene derivatives, as an electron donor have been developed for solar cell applications with an aim to enhance the PCE. As a result, the efficiency of polymer solar cells has been steadily improving, with PCE breaking the 9% barrier.
Recent advancements in most OPV technology relies on poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) as the hole-transporting layer. It is commonly adopted as the anode buffer layer in traditional polymer OPV cells due to its high transparency, high work function, smooth morphology and good conductivity. However, there are several disadvantages in using PEDOT:PSS as the hole-transporting layer, particularly with regard to its moisture permeability. As a film, PEDOT:PSS consists of doped conjugated polymer chains. The polymer grains are defined by PSS random coils with PEDOT chains attached. The area between the grains comprises of excess PSS. It has been suggested that the excess PSS can diffuse into the polymeric/organic layer, which will then possibly undergo a chemical reaction, which is undesirable.
In order to circumvent this, PEDOT:PSS has been replaced with metal oxides such as molybdenum (VI) oxide (MoO3), vanadium (V) oxide (V2O5), and tungsten (VI) oxide (WO3). The replacement of PEDOT:PSS was also mooted in order to improve the charge extraction by improving the built-in potential, hence elevating the overall PCE.
Most attempts to deposit MoO3 were performed via thermal evaporation means. This will be impractical for all solution-processable materials systems for large area electronics and optoelectronics devices. Furthermore, thermal evaporation involves the use of a vacuum chamber, which consumes lots of energy. Since simple large-scale processing is one of the prerequisites for practical large area production application, solution-processed MoO3 is a much more favorable proposition. Solution processability allows the use of methods such as spin-coating, slot die coating, inkjet printing, roll-to-roll manufacturing, for example.
Recently, a method for solution processable MoO3 films spin-coated from a nanoparticle suspension for organic electronics is developed. The MoO3 nanoparticles were pre-prepared and suspended with an undisclosed block copolymer. In another known method synthesizing metal oxide inks and films for electronic devices, the metal oxide, for instance, MoO3, was prepared by oxidizing the metal-ligand inks by heating or light irradiation or active oxygen exposure. The metal-ligand inks were prepared using metal complexes and organic solvents, which consist of carbonyl groups and/or hydroxyl groups. Unfortunately, both preparation methods are time consuming. Toxic organic compounds and solvents were also needed for these processes. In a separate investigation, a solution-processed MoO3 layer for OPV cells was prepared by spin-coating an aqueous MoO3 solution. The solution was prepared from (NH4)6Mo7O24 (ammonium molybdate) and hydrochloric acid solution. However, the ink is highly acidic, which can etch the indium tin oxide (ITO) electrodes.
Therefore, there remains a need to provide for alternative solution-based methods of forming molybdenum oxide films for optoelectronic applications.